Polyvinyl alcohol (PVA) is a polyhydroxy polymer and, consequently, a water-soluble synthetic resin having the general formula: ##STR1##
The largest application for PVA is in textile sizing. Other applications for PVA include its use in adhesives, paper coatings and as a polymerization aid. PVA has also been used as a component in soluble or bioabsorbable medical devices or surgical elements.
Various methods are described in the prior art for incorporating PVA into a medical device or forming such a device from PVA. For example, chemical attachment of a PVA coating to vascular protheses is described in U.S. Pat. No. 4,743,258.
Injection molding of PVA and modified PVA compounds has been suggested to form tampon applicators (U.S. Pat. No. 5,002,526) and components of bone graft implants (U.S. Pat. No. 4,863,472).
Solvent casting of PVA has been suggested for forming: a subsidiary device for suturing an intestine (U.S. Pat. No. 4,705,039); a bioerodible ocular device (U.S. Pat. No. 3,960,150); and a PVA-gel support pad from cross-linked PVA.
U.S. Pat. No. 3,922,434 describes a number of ways of applying a water soluble polymer such as PVA, to a paper substrate or carrier. One way of adding the polymer in connection with the manufacture of the carrier is to mix cellulose powder and pulverulent water soluble polymer, adding a small amount of water and then compression molding the mixture (See U.S. Pat. No. 3,922,434, column 4, lines 51-56).
PVA films oriented in two orthogonal directions may be produced in accordance with the methods described in U.S. Pat. No. 4,124,677.
The extrusion of a PVA rod which is chopped into pellets and then injection molded is described in U.S. Pat. No. 3,882,196.
In general, there are several problems associated with the molding of PVA. PVA is not easily molded as received from a supplier due to the fact that its melting temperature is above its degradation temperature. Therefore, a plasticizing agent, such as water, must be added. Only small amounts of water need to be added for the plasticizing effect to be experienced. However, adding only small amounts of water makes it very difficult to produce a homogenous mixture. A non-homogenous mixture results in an unacceptable product due to, among other things, non-uniform strength. Unfortunately, adding relatively large amounts of water causes the generation of numerous voids during the manufacture of a PVA film or object, again resulting in an unacceptable product.
Accordingly, it is an object of this invention to provide an improved method of forming soluble or bioerodible items from PVA.
It is a further object of the invention to provide a PVA molding process which overcomes the difficulties associated with known PVA molding techniques.